Mammals have evolved selective attention to extract useful sensory information from a complex world. Most previous neurophysiology research has modeled sensory representation and selective attention as independent, hierarchical processes: First, sensory systems passively filter input stimuli, and then attention systems select relevant information from the filtered output. The goal of this study is to test the alternative hypothesis that these two processes are tightly coupled at intermediate stages of sensory processing. If the filtering properties of sensory neurons are modulated by attention, then the number of possible computational strategies employed by attention will expand dramatically. This work will focus on characterizing the functional properties of neurons in primary auditory cortex under varying attention conditions. Findings using simple stimuli will be tested for their generality in natural behavior tasks. Patients with partial hearing loss and cochlear implants often complain of difficulty in auditory scene segmentation and hearing in noisy environments. This study will provide insight into the cortical processes required for performing these tasks successfully.